Express all physical quantities (such as force and velocity) in
terms of traveling-wave components. The traveling wave
components are called wave variables. For example, the force
on a mass is decomposed as
, where
is regarded as a traveling wave propagating toward
the mass, while
is seen as the traveling component
propagating away from the mass. A ``traveling wave'' view of
force mediation (at the speed of light) is actually much closer to
underlying physical reality than any instantaneous model.

We will not make much use of WDFs in this book, preferring instead
more prosaic finite-difference models for simplicity. However, we
will utilize closely related concepts in the digital waveguide
modeling context (Chapter 6).